Implement a process-local VFS to enable grouping together arbitrary
URIs passed via program arguments, DnD, or the file open dialog.
This VFS contains FivCollectionFile objects, which act as "simple"
proxies over arbitrary GFiles. Their true URIs may be retrieved
through the "standard::target-uri" attribute, in a similar way to
GVfs's "recent" and "trash" backends.
(The main reason we proxy rather than just hackishly return foreign
GFiles from the VFS is that loading them would switch the current
directory, and break iteration as a result.
We could also keep the collection outside of GVfs, but that would
result in considerable special-casing, and the author wouldn't gain
intimate knowledge of GIO.)
There is no perceived need to keep old collections when opening
new ones, so we simply change and reload the contents when needed.
Similarly, there is no intention to make the VFS writeable.
The process-locality of this and other URI schemes has proven to be
rather annoying when passing files to other applications,
however most of the resulting complexity appears to be essential
rather than accidental.
Note that the GTK+ file chooser widget is retarded, and doesn't
recognize URIs that lack the authority part in the location bar.
And store all direct thumbnailer output in the browser's cache--
low-quality thumbnails will always be regenerated, as is desired,
and we'll reload faster on devices where we don't store thumbnails.
This change improves latency at the cost of overall efficiency,
seeing as images with thumbnails will be spent cycles on twice.
Keeping this out-of-process avoids undesired lock-ups.
Moreover, embedded thumbnails can be fairly expensive to decode.
This will speed up sort changes, as well as simple reloads,
at the cost of an extra hash map from URIs to Cairo surface references.
It seems unnecessary to provide an explicit option to flush this cache,
as it may be cleared by changing either the directory or the current
thumbnail size.
At least the left and middle mouse buttons seem to behave similarly
in other programs and systems.
Context menus are opened on button release on Windows and with some
GTK+ widgets (popovers in GtkPlacesSidebar).
